If we view cities as densely populated areas surrounded by increasingly less populated and wilder land, then New York’s Jamaica Bay wetlands present this phenomenon in reverse. The 39-square-mile saltwater marsh at the far eastern edge of Queens and Brooklyn is a piece of nature engulfed by the country’s largest metropolitan area. Since the mid-1990s, the marsh, which hosts a multitude of fish and bird species, has been disappearing at an accelerating rate.

Scientists have a list of possible culprits. None – excess nutrients and the hardening of the bay’s shoreline, for example – is mutually exclusive. Indeed, the combination of several factors – what one scientist calls “a destructive synergy” – is likely behind the marsh’s degradation.

As such, the bay is something of a case study for the predicament of coastal wetlands in the United States and the world in general. Often, there’s not enough space for both wetlands and the sizable coastal population (53 percent, in the US) to coexist. Wetlands are drained, filled, and hemmed in by sea walls and bulkheads. Sediment deposition, necessary to counterbalance natural erosion, halts. With sea levels rising due to human-induced global warming, the wetlands, which could migrate inland in a pristine environment, drown.

City, state, and federal agencies are hashing out, and in some cases already implementing, various wetland-restoration strategies in Jamaica Bay. Proposed solutions include lowering nutrient influx and mimicking natural sedimentation by carting in sand. But the abiding question is, will these efforts address the underlying causes of marsh degradation?

Scientists and residents alike would like to avoid total marsh loss for a slew of reasons. The spongy soil, topped by tall grasses, buffers against storm surges. Many think that hurricane Katrina would have been less devastating had the Gulf Coast’s wetlands been intact and able to slow and absorb the storm surge. (Wetlands lining the Mississippi River could once soak up 60 days’ worth of floodwater, says the Environmental Protection Agency; what now remains can only hold 12 days’ worth.) A glance at a New York City flood-preparedness map shows that large swaths of Brooklyn and Queens directly behind Jamaica Bay are vulnerable to storm surges of only a few feet.

Wetland ecosystems also host a biodiversity rivaling that of coral reefs. They serve as a nursery for fish that, as adults, move to the open sea.

And they sequester carbon as peat, keeping climate-warming greenhouse gases out of the atmosphere.

At a July conference on wetlands in Brazil, scientists stressed the “carbon sink” function of wetlands. Worldwide, they estimate that, although they account for just 6 percent of the earth’s surface, the world’s wetlands – bogs, tundras, mangroves, swamps, and marshes – store between 10 and 20 percent of its terrestrial carbon. That’s an amount nearly equal the 771 gigatons already in the atmosphere. And if rising temperatures or more direct human disturbance lead to more wetland drying, scientists worry that the carbon released will further warm the planet.

Saltwater marshes require specific conditions to thrive: enough seawater to stay wet, but not so much as to drown. Sea levels have already risen nearly a foot along the Eastern Seaboard during the past 150 years, due partly to subsidence and partly to thawing polar regions. Locally, dredging has further altered tidal fluctuations by changing the “prism” of the bay, says Larry Swanson, director of Stony Brook University’s Waste Reduction and Management Institute on Long Island. The increased depth amplifies the tide, with highs and lows 8 to 10 inches above and below historical extremes, he says.

“We’ve totally altered the bay in a physical sense,” he says. “You can’t do that and not have some impact.” Add that to sea-level rise, and, at times, there’s 1.5 to 2 feet of extra water, compared with 100 years ago, he says.

Four wastewater-treatment plants empty into the bay. Although treated, the plants’ effluent is still high in plant nutrients like nitrogen, a byproduct of human waste. Between 1990 and 1995, nitrogen influx to Jamaica Bay doubled. (It has since decreased somewhat.) That’s when residents noted an acceleration of the marsh loss, an observation later corroborated by the DEC. Although scientists are quick to point out that correlation does not prove causation, many suspect that excess nitrogen – currently between 30,000 to 40,000 lbs. daily – is contributing to marsh degradation.

The DEC has asked New York City’s Department of Environmental Protection (NYCDEP) to address Jamaica Bay’s water quality. In response, the DEP proposed a list of solutions in the 2007 Jamaica Bay Watershed Protection Plan. Among other things, the list included upgrading wastewater-treatment plants to reduce the nitrogen load. But “there’s basically been a backsliding” since then, says Brad Sewell, senior attorney with the Natural Resources Defense Council in New York.

Citing the failure of limiting nitrogen in halting marsh degradation in places like Long Island Sound, a more recent NYCDEP white paper obtained by the Monitor seems to deemphasize this approach, which, it estimates, could cost city residents $6 billion.“Nitrogen levels have not been linked to marshland disappearance,” it states. “Upgrades will not attenuate this loss.”

But scientists have several working hypotheses explaining how excess nutrients may, in fact, harm wetlands. One is by making life too easy for the marsh grasses. “When plants get enough nutrients, they don’t produce as many roots,” says Dr. Turner. “When the storms come, they don’t have enough roots to hold the soil.”

Nutrients also spur algal blooms. When the algae dies, bacteria consume it and suck up oxygen. The story doesn’t end there, however. Another class of bacteria goes to work in these low oxygen areas. They use sulfates, a salt abundant in seawater, rather than oxygen to break down plant matter. And instead of exhaling carbon dioxide, they give off hydrogen sulfide. “That hydrogen sulfide is toxic to marsh plants in high concentrations,” says Alex Kolker, a research assistant professor at Tulane University in New Orleans. Spartina alterniflora, the dominant marsh grass here, can tolerate some sulfide, but “if you exceed the limits of its tolerance, it will die,” he says.

Help from oysters

Several restoration projects are already under way in Jamaica Bay. A pilot project is in the works to reintroduce oysters, absent since the 1930s. A single adult oyster can filter up to 50 gallons of water daily, clearing algae from the water.

“We know we can get them to survive,” says John McLaughlin, NYCDEP’s director of ecological services. “The next step is, can we get them to reproduce.”

Beginning in 2004, government agencies including the National Park Service and the Army Corps of Engineers planted spartina on hassocks fortified with dredged sand. Four years later, the restoration effort is considered a success. But ongoing losses are greater than what’s being restored. Without addressing the underlying causes of degradation, restoration efforts may not be sustainable in the long term, says Sewell.

Given the accelerating loss, some say there’s little time to waste. “We can’t just afford to let it go and not try to take all reasonable actions to keep it healthy,” says Mr. Swanson. “It may cost us in the short term, but [it’s] worth it in the long term.